Electric utilities and other organizations are responsible for supplying an economic, reliable and safe source of electricity. Three major components are employed in an energy delivery system to provide the electricity to the end user, the generator, the transmission line and the transformer.
The transformer is a device that changes voltage or a power angle. Generally, voltage from the generator is a lower voltage than used by the transmission lines that transmit the electricity to the end user. Furthermore, the voltage used by the end user is much lower than voltage used by the transmission lines. Thus, one exemplary purpose of the transformer is to couple elements of an energy delivery system that operate at different voltages.
Transformers come in many different sizes, shapes and constructions. Typically, transformer rating corresponds to the capacity of the transformer. The rating is typically specified as the product of the maximum voltage and current, as measured from one side of the transformer, that the transformer is capable of converting at a particular operating condition. Such operating conditions include temperature and/or altitude. For example, a 500/230 kV transformer may be rated at 300 MVA (3,000 kilo-volt-amps) when operating at sea level and at 65° Celsius rise above ambient. Transformers may be constructed as separately insulated winding transformers or as auto transformers (where the low voltage winding is a portion of the overall or high voltage winding). Both designs occur as single phase or multiple phase transformers. The operating voltages, ratings and winding types of transformers employed in the industry, well known to one skilled in the art, are too numerous to describe in detail here other than to the extent necessary to understand the present deficiencies in the prior art.
All transformers, independent of size, rating and operating voltage, have several common characteristics. First, the transformer is constructed from one or more windings, each winding having a plurality of individual coils arranged and connected in an end-to-end fashion. In some transformers, the winding is made by wrapping a wire around a laminated solid member, called a core. Alternatively, there may be no core. However, in all transformers, the individual winding turns must be electrically isolated from each other. An insulation material is wrapped around the wires such that when the plurality of coils are made, the metal wires of each winding are physically and electrically separated, or insulated, from each other. Insulation materials wrapped around the windings may vary. Paper, impregnated with oil, is often used. Other types of transformers may use only paper, or may use another suitable material such as a “polymeric compound.”
Maintaining the electrical insulation within and/or between the windings is absolutely essential for the proper operation of a transformer. In the event that the electrical insulation is breached, such that electricity passes from one winding coil across the breach to another winding coil, special protective devices will operate to disconnect the transformer from the electrical system. The devices, by removing electricity applied to the transformer, interrupt the undesirable current flow through the insulation breach to minimize damage to the transformer. This condition is commonly referred to in the industry as a “transformer fault.”
Transformer faults are undesirable for at least two major reasons. First, end users may become separated from the energy delivery system, thereby loosing their electrical service. Second, transformer faults may result in large magnitudes of current flow, known as fault current, across the breach and through the transformer windings. Also, faults occurring on the energy delivery system at locations relatively close to the transformer may result in large fault currents flowing through the transformer. Often, fault current may be orders of magnitude greater than the highest level of normal operating current that the transformer was designed to carry. Such fault currents may cause severe physical damage to the transformer. For example, a fault current may physically bend portions of the transformer winding (winding deformation) and/or move the windings out of their original position in the transformer (winding displacement). Such winding deformation and/or displacement can cause over-voltage stresses on portions of the winding insulation and exacerbate the process of the naturally occurring deterioration of the winding insulation that occurs over a period of time. The fault current may further increase damage to the insulation, or damage insulation of adjacent windings, thereby increasing the magnitude and severity of the fault. In the most extreme cases, the fault current may cause an ignition in the transformer oil, resulting in a breach of the transformer casing and a subsequent fire or explosion.
Therefore, it is desirable to ensure the integrity of the transformer winding insulation. Once a transformer fault occurs, it is usually too late to minimize transformer damage and to reduce the period of electrical outage. The electric utility industry takes a variety of precautionary steps to ensure the integrity of winding insulation in transformers. One important precautionary step includes periodic testing of the transformer. Various tests are used to predict a probability of a future fault. One test commonly employed in the industry to detect winding deformation and/or displacement is the low voltage impulse test.
Prior art low voltage impulse tests present many unique problems. One significant problem is that a precise, repeatable input testing signal or pulse of known energy content to be sufficient for the test must be applied to the input terminal of the tested transformer winding when prior art frequency response analysis techniques are used to measure the frequency response of the transformer winding. If the applied input test signals/pulses are not identical to each other, the resultant characteristic signature of the tested transformer windings will not be accurate. For example, the prior art has no objective test accuracy or bandwidth limit analysis, so an unknown pulse at the input will compromise the test result without detection. In addition, the time delay between pulse applications for the prior art should be constant to prevent random distortion of the input pulse which affects the characteristic signature. For example, if the pulse intervals are not constant, the energy storage remaining in the transformer winding configuration will be different between pulses, thus altering the load impedance of the transformer and therefore, changing the parameters (frequency energy content) of the applied pulse. Furthermore, test signal/pulse generators or test pulse generators capable of providing such exact and repetitive input signals or pulses are expensive.
One technique of testing for an off-line, or de-energized, transformer winding deformation compares the characteristic signature [H(f)] for a winding over time. Changes in H(f) indicate winding deformation and/or displacement. H(f) is determined using a unique computational method fully described in the U.S. utility patent to Coffeen entitled, “SYSTEM AND METHOD FOR OFF-LINE IMPULSE FREQUENCY RESPONSE ANALYSIS TEST,” having U.S. Pat. No. 6,369,582, filed on May 3, 2001, and issued to patent on Apr. 9, 2002, which is incorporated herein by reference in its entirety.
To derive H(f) for a winding under the U.S. Pat. No. 6,369,582 patent to Coffeen, a suitable number of input pulses or signals, provided by a pulse or signal generator, are applied to the winding. When this test is performed on a transformer, the transformer is offline. That is, the transformer has been disconnected from the electric system and is in a fully discharged, de-energized state.
After the test pulses or signals are applied to the winding, the auto-spectral density (Gxx) is calculated. Gxx is defined by the complex conjugate of the fast Fourier transform (FFT) of the input pulse or signal times the FFT of the same impulse or signal. The cross-spectral density (Gxy) is also calculated. Gxy is defined by the complex conjugate of the FFT of the input pulse times the FFT of the output pulse. The characteristic signature [H(f)] for the winding equals the average of the Gxy's divided by the average of the Gxx's for the respective pairs of input and output pulses or signals. Preferably, input pulses or signals are slightly different, or even very different, from each other.
Once H(f) is determined for the winding, the determined H(f) is compared with another earlier determined H(f) for that winding. Winding deformation and/or displacement can be determined by comparing the most recent H(f) with an earlier determined H(f).
Another technique of testing for transformer winding deformation also compares the characteristic signature [H(f)] for a winding over time. Here, the transformer is on on-line, or energized. The characteristic signature H(f) is determined using a unique computational method fully described in the U.S. utility patent to Coffeen entitled, “SYSTEM AND METHOD FOR ON-LINE IMPULSE FREQUENCY RESPONSE ANALYSIS TEST,” having U.S. application Ser. No. 09/848,921, filed on May 3, 2001, which is incorporated herein by reference in its entirety.
To derive H(f) for a winding under the Ser. No. 09/848,921 patent application to Coffeen, input pulses or signals that result from events out on the electric system propagate through the winding. Accordingly, the transformer is on-line. That is, the transformer remains connected to the electric system and is in a fully energized state.
Incoming voltage pulses due to abrupt changes in current or voltage originating elsewhere on the energy delivery system are detected. An output pulse is detected after the input pulse has propagated through the monitored winding. Spectral densities are determined from these detected input and output pulses. However, the electrical characteristics of these pulses, such as the current, voltage, frequency, wave shape and/or energy are unpredictable and vary randomly from pulse to pulse. Not all pulses will have sufficient energy to generate useable information that can be used to calculate spectral densities for all the frequencies of interest. Some pulses may have sufficient energy so that the spectral densities for all of the frequencies of interest are calculated. Other pulses will have sufficient energy in some frequencies so that spectral densities for some portions of the frequencies of interest are calculated. Accordingly, the on-line winding test unit monitors a winding and records the input and output pulses. Logic is executed that analyzes the input and output pulses by way of spectral densities to identify useable H(f) data, which is further processed to build a characteristic signature H(f) from the pieces of usable data. When a sufficient record of useable H(f) data portions are accumulated, a complete characteristic signature, H(f), for the monitored winding is constructed. Winding deformation and/or displacement can be determined by comparing the most recent computed H(f) with an earlier determined H(f).
However, the above-described systems that detect transformer winding deformation and/or displacement requires historical H(f) information that is compared to the currently determined H(f) information. Therefore, it is desirable to have a valid and reliable testing system and method that does not require historical H(f) information.